"We not trying to discourage people from resistance training," said study author Leslie Willis, clinical research coordinator at Duke University Medical Center and an exercise physiologist.

Previous studies have shown that resistance training has many benefits, including improving blood sugar control, she said, but the effects of it on fat reduction have not been conclusive.

The new study, published Dec. 15 in the Journal of Applied Physiology, compared resistance training to aerobic exercise to determine which is best for weight and fat loss.

The new study results suggest for people short on time, focusing on aerobic exercise is the best way to lose weight and fat, Willis said.

Willis' team assigned 234 middle-aged men and women, all overweight or obese, to one of three groups for the eight-month study. The resistance-training group worked out three times a week, with instructions to exercise about three hours total. They used eight different weight machines.

The aerobic group put in about 12 miles a week on elliptical machines or treadmills, putting in about 133 minutes a week, or about 2 1/4 hours.

The combination group worked out three days a week, putting in the combined effort of the resistance training and the aerobic groups.

In all, 119 finished the study. Those who did aerobic exercise or the combination reduced total body mass and fat mass more than those in the resistance group, but they were not substantially different from each other, Willis said.

For instance, the aerobic only group lost 3.8 pounds and the combination group lost 3.6 pounds.

The combination group did notice the largest reduce in waist circumference. A large waist (over 35 inches in women, over 40 in men) is a risk factor for heart disease and other problems.

The combination group ''did double the time commitment without significantly improving the result over the aerobic group alone for fat mass," Willis noted.

"If fat mass is something a person wants to target, I would say your most time-efficient method would be to focus on the cardiovascular exercise," she said.

"Resistance training did increase lean mass, but it doesn't change fat mass, so the pounds didn't change," she said.

Dr. Timothy Church, director of preventive medicine research at Pennington Biomedical Research Center in Baton Rouge, La., said experts have known that "aerobic exercise really helps with weight loss."

However, he said, the study results are no reason to dismiss resistance training. People lose muscle mass as they age, he said, and resistance training, which helps maintain muscle strength, can help with quality-of-life issues.

It can help people perform such small but important everyday tasks as lifting their grandchildren and getting luggage into overhead bins on airplanes, he said.